1. Technical Field
The present invention relates to a high-stability piezoelectric oscillator which includes a double thermostatic oven.
2. Related Art
Generally, for various piezoelectric oscillators which have been used as a reference signal source for a GPS frequency generating device, a base station for a portal terminal, or the like, piezoelectric oscillators (crystal oscillators), each of which has excellent resonance characteristic and uses a crystal having high frequency stability, have been used. In recent years, a crystal oscillator, which has been used for the various devices or base stations, is required to have a stable oscillating output even under poor environmental conditions where the ambient temperature varies or the temperature and humidity is high. Therefore, as an oscillator which can satisfy this request, an oven controlled crystal oscillator (hereinafter, referred to as ‘OCXO’) has been mainly used.
FIG. 7 shows an OCXO according to the related art. This OCXO includes upper and lower printed circuit boards 101 and 102 (circuit mounted boards), a crystal resonator 103 whose a lead terminal 103a is connected to the upper printed circuit board 101 and which is accommodated in a thermostatic oven 104, a thermo-sensitive element 105 that is installed in the thermostatic oven 104, temperature control circuit components 106 that are mounted on the lower printed circuit board 102, oscillating circuit components 107 that are mounted on the upper printed circuit board 101, pins 108 that pass through the upper and lower printed circuit boards 101 and 102 so as to electromechanically connect the upper and lower printed circuit boards 101 and 102 to each other, and an outer casing 110 that surrounds the upper and lower printed circuit boards 101 and 102, the crystal resonator 103, the thermo-sensitive element 105, the temperature control circuit components 106, the oscillating circuit components 107, and the pins 108. The outer casing 110 has a lower case 111 that forms a bottom plate, and an upper case 112 that surrounds a space formed on the lower case 111, including the above-mentioned various constituent elements. A lower end portion of each of the pins 108 passes through the lower case 111 to protrude downward. As a result, each of the pins 108 serves as a connection unit when the oscillator is mounted on a mother board (not shown).
The thermostatic oven 104 is controlled by means of the thermo-sensitive element 105 and the temperature control circuit 106 such that a predetermined temperature can be maintained in the thermostatic oven even if the ambient temperature varies, and thus a predetermined temperature is maintained in the crystal resonator 103 in the thermostatic oven 104. The upper and lower printed circuit boards 101 and 102, to which the thermostatic oven 104 is attached, are electromechanically connected to each other by means of the connection pins 108 so as to be firmly fixed. In this state, the upper and lower printed circuit boards 101 and 102 are supported on the lower case 111 by means of the pins 108. The lower case 111 is covered with the upper case 112, so that a shielding effect or an effect of preventing the heat from radiating from the thermostatic oven can be achieved. For the frequency stability (frequency-temperature characteristic) of the OCXO having the above-mentioned structure with respect to the variation of the ambient temperature, frequency stability of about 10−8 can be obtained.
As oscillators for obtaining oscillating output having higher stability than the OCXO, for example, in JP-UM-A-60-142536, an oscillator having the following structure has been disclosed. According to this oscillator, a crystal resonator and an oscillating circuit are accommodated in a heat-insulated case composed of a thermos bottle, and a heat capacity of the thermostatic oven is increased so as to obtain higher temperate stability. Further, in JP-A-62-003527, an oscillator having the following structure has been disclosed. According to this oscillator, higher temperature stability is obtained by using a double thermostatic oven that includes an inner oven and an outer oven. Furthermore, in JP-A-2003-309432, a high-stability piezoelectric oscillator having the following structure has been disclosed. According to this high-stability piezoelectric oscillator, a thermostatic oven, in which a piezoelectric vibrator and oscillating circuit components are accommodated, is disposed in a metallic case.
FIG. 8 is a diagram illustrating a structure of a high-stability piezoelectric oscillator using a double thermostatic oven having an inner oven and an outer oven.
This high-stability piezoelectric oscillator includes an oscillating circuit unit 114 that has a piezoelectric vibrator 112 and oscillating circuit components 113, an inner oven 115 (having an inner lower case member 115a and an inner upper case member 115b) that accommodates the oscillating circuit unit 114, an outer oven 120 (having an outer lower case member 120a and an outer upper case member 120b) that accommodates the inner oven 115, a base printed circuit board 125 that supports the oscillating circuit unit 114, the inner oven 115, and the outer oven 120, and an outer metallic case 130 (having a lower metallic case member 130a and an upper metallic case member 130b) that covers the outer oven 120 and the base printed circuit board 125. The inner printed circuit board 116 is disposed in the inner oven 115 such that it supports the piezoelectric vibrator 112 and the oscillating circuit components 113. In addition, the outer printed circuit board 121 is disposed in the outer oven 120 such that it supports the inner oven 115, the heater 122, and a temperature control circuit component of the heater 122. Further, the base printed circuit board 125 supports a heater 126 and a temperature control circuit component of the heater 126.
In the high-stability piezoelectric oscillator according to the related art, a bonding portion 131 between an outer circumferential portion of the lower metallic case member 130a that forms a portion of the outer metallic case 130 and a skirt portion of the upper metallic case member 130b is not completely sealed. Further, in each of the inner oven 115 and the outer oven 120, a bonding portion between the upper and lower case members is not completely sealed.
For this reason, outside air may easily permeate into a minute gap of the bonding portion 131 of the outer metallic case 130. If the vapor contained in the outside air having permeated in the outer metallic case 130 is absorbed by the base printed circuit board 125 made of glass epoxy or the like, the base printed circuit board causes deformation, such as expansion, warping, or the like. As a result, a load is applied to the temperature control circuit components of the heater 126 that are mounted on the base printed circuit board 125 because of the stress generated by the deformation of the base printed circuit board, a constant thereof may be easily changed. Specifically, wiring patterns formed on the board deform, so that a floating capacitance may vary. Or, a stress is applied to electronic components, such as a capacitor or the like, forming an oscillating circuit, so that an element value may vary.
Further, in the outer oven 120 and the inner oven 115, each bonding portion is not sealed. Therefore, an outside air containing vapor permeates into the respective ovens, causing the inner printed circuit boards 121 and 116 to also be deformed, which results in a load being applied to each of the temperature control circuit components of the heaters mounted on the respective printed circuit boards or the piezoelectric oscillator 112. As a result, variation is generated in a frequency output by the oscillator and thus reliability is lowered. In a high-stability piezoelectric oscillator which requires frequency stability of 10−8 to 10−9 order, even a small amount of frequency variation can cause severe damage. In recent years, higher frequency stability of 10−10 order has been required.
In order to satisfy a request for high frequency stability, a method of airtightly sealing the bonding portion 131 of the outer metallic case 130 through brazing or resin sealing has been generally adopted. As shown in FIG. 8, ideally, it is expected that frequency stability of 10−10 order can be achieved by forming a thermostatic oven (oven) with a double structure and sealing the bonding portion of the outer metallic case. However, actually, the frequency is not stabilized, and only frequency stability of 10−9 order is achieved. That is, according to a high-stability piezoelectric oscillator using the double thermostatic oven, it is considered that frequency stability of 10−10 order can be obtained because the structure is optimal in obtaining oscillation output with high stability. However, when the oscillation output is output with extremely high stability, the high-stability piezoelectric oscillator is affected by permeation of the outside air, variation in assembly at the time of manufacture, characteristic variation of components, or the like, causing possible variation in a frequency-temperature characteristic.
In the above-mentioned high-stability piezoelectric oscillator using the double thermostatic oven, even when the outer metallic case is airtightly sealed so as to prevent the outside air from permeating therein, it is not possible to achieve the ideal frequency stability of 10−10 order.
The following reasons are given as to why the above-mentioned problems occur. The inventors have discovered a phenomenon that an oscillation frequency varies in response to the temperature variation of the outside air, and a phenomenon that the frequency variation becomes when the bonding portion of the outer metallic case is not airtightly sealed. Therefore, it is assumed on the basis of the discovery that in a case in which the bonding portion of the outer metallic case is sealed, the expansion or contraction of air sealed in the case generated due to the temperature variation of the outside air causes the frequency to be unstable. In addition, in the high-stability piezoelectric oscillator using the double thermostatic oven, sealing the bonding portion of the outer metallic case airtightly causes the ideal frequency stability of 10−10 order not to be achieved. However, since it is not possible to accept a disadvantage occurring when the bonding portion of the outer metallic case is not airtightly sealed, development of a high-stability piezoelectric oscillator has been demanded in which it is possible to achieve frequency stability of 10−10 order without being affected by the temperature variation of the outside air while airtightly sealing the bonding portion.